1. Field of the Invention
The present invention deals with a lighting unit designed for functional similarity to an incandescent light source, and more particularly with a lighting unit in which the principal source of light is an arc discharge lamp supplemented by a standby filamentary light source, and which includes a compact "high frequency" power supply unit for supplying the needed energization from a conventional 110 volt 60 hertz source.
2. Background of the Invention
The present invention is an outgrowth of earlier efforts to produce an energy efficient and comparatively low cost replacement unit for the incandescent lamp. The incandescent lamp converts most of the electrical energy supplied into heat, a small percentage always less than 10% being converted into visible light. With the cost of energy rising, a need has arisen for a lighting unit which converts electrical energy into light with greater efficiency. Known lighting units such as fluorescent units have had double or triple the lighting efficiency of an incandescent light. A property of such devices, whih has limited their more general application, has been the high initial cost of the ballast for powering such devices and their elongated configuration. Another possible alternative has been the high pressure arc discharge lamp having up to six times the efficiency of an incandescent lamp. High pressure metal vapor ac discharge lamps have been available in high power units requiring costly power supplies, restricting their use to street lighting and commercial as opposed to home lighting. Recently, as disclosed in U.S. Pat. No. 4,161,672, smaller, low wattage, metal halide lamps having efficiencies approaching those of the larger size have been invented. Such lamps are a potential energy efficient replacement for the incandescent lamp provided that convenient low cost provisions can be made for standby illumination and for supplying the diverse electrical requirements for the two light sources.
A prior solution to the problem of a replacement unit for an incandescent lamp is contained in the U.S. application Ser. No. 29,954, filed Apr. 13, 1979 of William Peil, entitled "Lighting Network Including a Gas Discharge Lamp and Standby Lamp" and assigned to the Assignee of the present invention, that application being a continuation-in-part of an earlier application, Ser. No. 909,300, filed May 24, 1978.
The power supply of the present lighting unit represents an outgrowth of earlier high frequency power supplies in which a ferrite transformer, normally controlled or non-saturated operation and a transistor switch are the most significant elements. Such power supplies have been termed static inverters in deference to the fact that "dc" quantities are converted to ac through static or non-moving parts. Patents dealing with inverters of this class and ferrite transformers having the saturation avoidance feature include the U.S. Pat. Nos. 3,914,680, 4,002,390 and 4,004,251.
The U.S. application Ser. No. 974,253 deals with a power supply for supplying high voltage using ac harmonic ring-up of the inverter voltage.
The U.S. application Ser. No. 139,946 deals with a ferrite transformer having a saturation avoidance feature of the type herein employed.
The U.S. application Ser. No. 028,405 deals with a ferrite transformer having a modified construction, also having saturation avoidance features, which is of generally less convenience for the present application, but electrically similar.
The U.S. Pat. No. 4,202,031 deals with other static inverter configurations generally applicable to supplies producing high voltage energy.
In the general art of arc lamps and controls therefor, it has been suggested that a resistive incandescible filament be connected in series with an arc lamp to serve as a steady running ballast, the current causing the filament to incandesce. See, for example, Thouret U.S. Pat. No. 3,048,741. McLellan U.S. Pat. No. 4,100,462 shows such a steady-on filament in series with a fluorescent lamp. The steady-on series filament results in shorter total lamp life because a filament at incandescent temperatures has a limited number of operating hours, these being foreshortened by the high starting currents, as described in the book "High Pressure Mercury Vapor Lamps and Their Applications" by Elenbaas, Philips Technical Library, 1965, in the first full paragraph on page 140. The steady-on filament further consumes significant power during the running of the arc lamp and precludes the realization of a very high efficiency (high lumens per watt) for the composite incandescent/arc light source.
Further, it has been suggested that a resistive incandescible filament may be physically associated with an arc discharge lamp but selectively energized by controlled switching means so as to be excited when the arc lamp is in a starting and/or a failed mode. Examples of such prior art are Lake U.S. Pat. No. 3,527,982, McNamara U.S. Pat. No. 3,517,254, the copending application Ser. No. 29,954 identified above, and Haymaker et al U.S. Pat. No. 3,536,954. In those arrangements, however, (excepting Lake wherein part of the filament is shunted after arc starting) the filament serves no ballasting function, and the arc lamp must be ballasted by separate elements or controlled conduction devices. This leads to a greater number of parts, cost, complexity and volume for the control system as a whole.
Finally, in arc lamp systems involving no incandescible filament at all, it has been taught broadly to generate and apply high voltage starting energy to the arc lamp terminals, and then to turn off the starting energy in response to the establishment of the arc. Snyder U.S. Pat. No. RE. 29,204 and Elms U.S. Pat. No. 4,103,209 disclose such arrangements in control systems for energizing arc lamps from alternating electric power sources.